The capability of seating multiple riders on a bicycle has often been provided by two or more seats mounted serially in-line (front-to-back) between two wheels of the bicycle. This arrangement for two riders is termed "tandem", and generally allows only the front rider to steer the bicycle. A key purpose for riding the same bicycle with another rider is the opportunity for conversation during travel. Such communication is difficult in a serial seating arrangement because the front rider must focus on the road ahead for steering purposes and the rear rider cannot easily hear the front rider because the front rider faces away from the rear rider. Lack of effective communication may also be a significant safety problem because rear riders need to be instructed as to which way to lean when cornering and whether to brake or speed up in various situations controlled by the front rider. Furthermore, rear riders cannot conveniently see what is on the road ahead because their vision is blocked by the front rider. Having no control of direction, unable to effectively converse, and being blocked from forward viewing truly makes a rear rider on an in-line tandem bicycle a second class cyclist.
To solve the communication problem, side-by-side or parallel seating is a preferred arrangement. Motorcycle side cars, for example, provide such an arrangement. Three-wheeled cycles may have side-by-side seating, although tricycles are notoriously unstable during cornering. More recently it has become popular to connect in a side-by-side fashion two independent bicycles to form a quadracycle. Such a combination may include two single-rider bicycles or two in-line tandem bicycles, serving two or four riders, respectively.
Toppling stability of a quadracycle is a function of the spacing of the wheels and the height of the center of gravity of the structure including riders. The greater the spacing of wheels and the lower the center of gravity, the greater the toppling stability. Standard bicycles have high centers of gravity and short wheelbases which, when tied together as a quadrcycle, encourage toppling. Long wheelbase recumbent bicycles, which enable the riders to sit closer to the ground because the riders' legs are extended forward more than downward, provide a low center of gravity. Another benefit of using recumbent bicycles for constructing quadracycles is that handlebars for steering purposes may be located under each rider's seat. Such an arrangement frees up the front of each bicycle for the inclusion of a steering linkage.
The forces applied to a quadracycle are predominantly gravitational forces, which tend to press each wheel of the bicycle downward against the ground. However, there are other forces also present during riding. For example, when cornering, the bicycle frames may tend to tilt from their normal upright orientation. Also during cornering, road friction and centrifugal forces may cause the bicycles to spread apart more at one end than the other end. Tilting and spreading forces must generally be resisted in order for steering to function properly. Prior art quadracycles therefore have multiple rigid connections between bicycles that generate a substantially rigid structure. Such a rigid structure substantially resists deflection from all applied forces and therefore fails to maintain all four wheels on an uneven road surface at all times. Maintaining all wheels in contact with the road surface is essential in order to steer properly and to distribute weight proportionally to each of the four wheels.
It is important for quadracycle steering that side-by-side bicycle frame planes remain substantially parallel. That is, when traveling forward, each bicycle has a substantially upright orientation and each of its two wheels are substantially in-line, thereby occupying two side-by-side upright planes. Even when road surface unevenness is encountered, the substantially upright plane of one bicycle ideally remains substantially parallel to the substantially upright plane of the other bicycle for steering purposes.
It is believed that providing a side-by-side bicycle connecting structure that is rigid enough to maintain parallel upright bicycle frames while providing the vertical compliance necessary to maintain all four wheels in contact with the ground is a problem in quadracycle design that has not been effectively or economically addressed. Independently spring loading of each wheel is one solution, as is done in some automobile suspensions. Spring loading generally requires that one part can move relative to another with a spring in between. Hinges, guide posts, vibration damping, or other moving part restraints are needed in addition to springs, all adding components, and therefore, manufacturing complexity.
Another problem with quadracycles is a need for coordinated steering of two front wheels. For straight ahead travel, both steered wheels should be substantially parallel to the parallel bicycle frames. A great amount of tire frictional drag occurs when steered wheels are aimed incorrectly. For turning corners, the angle of the steered wheel closest to the center of turning should ideally be angled somewhat more than the steered wheel furthest from the center of turning. A problem with such steering is providing a common mechanism for both steered wheels that accomodates the vertical compliance of bicycle frames while maintaining proper wheel alignment.
An object of the present invention is to provide a quadracycle having a single rigid connection member between two bicycles which maintains bicycle frames substantially upright and parallel while providing sufficient bicycle frame vertical compliance to allow all four wheels to continually contact uneven road surfaces during travel, without the use of springs.
Another object of the present invention is to provide a steering linkage sufficiently rigid for steering purposes yet adaptable to relative bicycle frame misalignment when uneven road surfaces are encountered.